1. flag day
A software change that is neither forward- nor
backward-compatible, and which is costly to make and costly
to reverse. “Can we install that without causing a
flag day for all users?” This term has nothing to do
with the use of the word flag to mean
a variable that has two values. It came into use when a
change was made to the definition of the ASCII character set
during the development of Multics.
The change was scheduled for Flag Day (a U.S. holiday),
June 14, 1966.The change altered the Multics definition of ASCII from the
short-lived 1965 version of the ASCII code to the 1967 version (in draft at
the time); this moved code points for braces, vertical bar, and
circumflex. See also backward combatability. The
Great Renaming was a flag day. [Most of the changes were made to files stored on
CTSS, the system used to support
Multics development before it became self-hosting.]
[As it happens, the first installation of a
commercially-produced computer, a Univac I, took place on
Flag Day of 1951 —ESR]
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Definitions of flag day 
